The present invention relates to an improved process for the regeneration of catalysts for the gas-phase reduction of aromatic nitro compounds to aromatic amines.
The most important processes for the industrial preparation of aromatic amines are reductions of the corresponding nitro compounds in the gas phase. For example, aniline, an intermediate for rubber chemicals and for methyenediphenyl diisocyanate (precursor for polyurethanes), is produced in large quantities mainly by reduction of nitrobenzene with hydrogen in the gas phase. According to a newer process, it is also possible to prepare aniline by reduction of nitrobenzene with natural gas in the presence of catalysts containing copper chromite, and steam (see, for example, EP-OS (European Published Specification) No. 0,087,690).
With such reductions in the gas phase, the Catalyst loses activity over the course of time, that is to say there is a gradual reduction in the conversion of nitro compound, frequently associated with a decrease in selectivity. Although the catalysts used industrially can be regenerated, the intervals between regenerations then become, of necessity, increasingly short. There are reports on regeneration and the possibility of regeneration in, for example, Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd edition, Volume 5, page 21 (1979) and in Ullmanns Encyclopadie der technischen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry), 4th edition, Volume 13, page 565 (1977). Treatment with air (see, for example, U.S. Pat. No. 2,891,094) or with air/stream mixtures (see, for example, U.S. Pat. No. 2,292,879), followed by a reducing after-treatment, is customary for the regeneration of catalysts for the reduction of nitrobenzene. Treatment with air/stream mixtures is indicated for the abovementioned reduction process using natural gas.
Interruptions in the production of amine caused by catalyst regeneration not only adversely affect the economics, but also represent a serious interruption in the distillation stages on continuous working up of the mixture which is obtained after the reduction. Similar problems result with the supply of hydrogen, which is frequently effected continuously by steam reforming. It is then necessary during the regenerating phase to combust the hydrogen which has been made available but not used, or to deliver it at the caloric value, since temporary shut-down of the reformer is not possible, on the contrary, operation must continue at a reduced capacity (about 40% of the full load). Thus it would be possible to improve the economics of such reduction processes for the preparation of aromatic amines if it were possible to increase the lives of the catalysts.